This invention relates to improvements in electrical power assisted steering systems of the kind in which an electrical motor is adapted to apply an assistance torque to a steering component such as a steering column so as to reduce the driver effort required to control the vehicle.
In a simple electric power assisted steering system a torque sensor is provided which is arranged so that the level of torque in a steering column is measured. From this measurement a controller calculates the value of a torque demand signal which is indicative of the torque that is to be generated by an electric motor attached to the steering column. The motor applies a torque to the column of the same sense as that demanded by the driver and thus reduces the effort needed to turn the wheel.
A problem with this simple arrangement occurs in certain driving manoeuvres which excite a vehicle yaw mode transient responsexe2x80x94leading to so-called xe2x80x9cfish-tailingxe2x80x9d of the vehicle. These manoeuvres are typically the result of xe2x80x9cunsupportedxe2x80x9d driver actions on the handwheel such as rotational xe2x80x9cflicksxe2x80x9d where the drives applies a rapid handwheel angle change but does not follow it through with any substantial applied torque or perhaps releases the handwheel after initiating a rapid turn.
In such circumstances it is desirable that the handwheel returns to the central xe2x80x9cstraight-aheadxe2x80x9d position quickly and with a minimum amount of overshoot or oscillation. In general, however, geometric and inertial effects of the steering system contribute to a free mode yaw response that is lightly damped and quite oscillatoryxe2x80x94particularly at high vehicle speeds.
It is known in the art to overcome this problem by including a damping component within the torque demand signal that is used to drive the motor. This damping component in some sense mimics the mechanical phenomenon of viscous friction through software by generating a component of torque demand that is a function of the handwheel velocity. The damping component may generally increase in magnitude as a function of steering angular velocity from zero torque at zero rotational speed to a maximum at some arbitrary maximum speed. In effect, the damping component reduces the actual torque output by the motor, and hence the amount of assistance, in a particular instance when the velocities are high.
In accordance with a first aspect the invention provides an electric power assisted steering system comprising a steering mechanism which operatively connects a steering wheel to the roadwheels of the vehicle, a torque sensing means adapted to produce a first output signal indicative of the torque carried by a portion of the steering mechanism, a means for producing a second output signal indicative of the angular velocity of the steering wheel, an electric motor operatively connected to the steering mechanism, a signal processing unit adapted to receive the two signals and to produce a torque demand signal representative of a torque to be applied to the steering mechanism by the motor, and a motor drive stage adapted to provide a drive current to the motor responsive to the torque demand signal, and in which the torque demand signal includes a damping component that is dependent upon both the first output signal and the second output signal.
By making the damping component a function of torque as well as angular velocity of the steering column it has been found that the damping component does not become intrusive during xe2x80x9chands-onxe2x80x9d slalom manoeuvres yet the damping remains effective in improving yaw response in other circumstances such as a steering flick.
The magnitude of the damping component preferably generally increases over a range of steering velocity values bounded by a first velocity and a second, higher, velocity. Thus, as steering velocity is increased more damping is introduced. The first velocity may correspond to zero column velocity. The second velocity may correspond to the maximum expected column velocity or some other arbitrarily selected value. Alternatively, a deadband may be provided whereby the damping component value remains at or about zero over a range of velocities bounding zero velocity. The width of this deadband may be varied in use, and may for example be varied as a function of vehicle speed or another measured parameter.
The magnitude of the damping component may generally increase linearly as a function of column velocity over the whole or a part of the range of values. Thus, the value of the damping component may become generally higher as the angular velocity of the steering wheel increases. However, a non-linear relationship may exist between velocity of the steering wheel and the damping component value.
In the preferred arrangement the rate of increase of the magnitude of the damping component between the first and second values preferably decreases as a function of applied torque. The damping component, in one arrangement, may be produced by generating a scaling value that is a function of torque, generating an intermediate damping value that is a function of column velocity, and multiplying the two values together to produce the damping component.
The scaling value may vary from a maximum value at zero applied torque to a minimum value at a predetermined maximum applied torque. In this case, for torque values at or above the maximum then a zero valued damping component will be produced.
The scaling value may be adapted to be substantially zero valued over a range of measured torque values bounding zero torque. This provides a deadband either side of zero torque about which for a given steering wheel velocity a maximum damping component is produced, improving steering feel for high speed on centre manoeuvres.
In a further refinement the width of the deadband may be varied as a function of the speed of the vehicle to which the steering system is fitted. A measurement of vehicle speed may therefore be provided to a third input of the signal processor.
The torque demand signal may include an assistance torque signal that is a function of torque in the steering mechanism. The assistance torque signal may generally increase with increasing torque applied by the driver. The signal processor may be adapted to produce the torque demand signal by combining the damping component with the assistance torque signal. Preferably the damping component is subtracted from the assistance torque signal.
The assistance torque signal may be a function of other variables such as vehicle speed.
The signal processor may calculate the value of the damping component for any given combination of torque and steering wheel velocity from entries in a look-up table. In this case, each or specific combinations of steering velocity and driver input torque will access a specified value stored in the table.
In a preferred alternative, the value of the damping component may be derived by entering the velocity, torque and optionally vehicle speed values into a suitable equation.
Whilst the provision of a damping component that is a function of torque as well as angular velocity of the steering column provides appropriate levels of damping during xe2x80x9chands-onxe2x80x9d slalom manoeuvres it can, in certain circumstances, induce unwanted torque variations in the steering column shaft. For example, when a high frequency driver applied torque is generated, or the column kicks back due to impacts on the road wheels, the torque dependent damping component can interact with the applied torque setting up an unpleasant oscillation. Thus, the driver applied torque can affect the damping torque which in turn affects the driver applied torque and so on.
In a refinement, to ameliorate such an effect the damping component may be filtered to remove high frequency variations in the damping component caused by high frequency changes in the column torque. Thus, the system may include limiting means adapted to limit the rate of change of the damping component due to corresponding changes in column torque to a predetermined maximum rate.
Preferably, the rate limiting means may comprise a filter. This may comprise a lower pass filter, when in one arrangement may have a cut-off frequency of approximately 3 Hz (Hertz).
In a most convenient arrangement, where the damping component comprises the product of a scaling value that is a function of torque and an intermediate damping value that is a function of the column velocity, the limiting means may be arranged to limit the rate of change of the scaling value over time. The scaling value may be low-pass filtered prior to multiplication by the intermediate damping value to generate the damping component.
The low-pass filter may be a frequency domain filter but may be of any known kind, typically a discrete digital filter implemented on a microprocessor. Of course, any processing of the scaling value which limits the maximum rate of change of the scaling value over time could be employed.